This invention relates to an improved apparatus for making a web of uniform thickness across the web width.
As illustrated by U.S. Pat. No. 3,767,346 to Mihalik, an apparatus having a plurality of short heat exchangers movably mounted in a groove in a lip of the apparatus for movement along the length of the lip, for varying the lip temperature at very small selected areas, is known. The heat exchangers include inlet and outlet conduits for a medium such a oil which may be recirculated to its source for heating or cooling The apparatus also includes manually-operated, lip gap adjustment members.
In addition, as exemplified by U.S. Pat. Nos. 3,761,553 to Richardson, 3,920,365 to Mules, and 4,332,543 to Fulton et al, an apparatus having a plurality of thermal energy sources, disposed across the width of an apparatus, and providing for localized changes in web thickness based upon localized viscosity changes, is known. The web thickness is decreased or increased in response to induced localized thermal gradients. The Richardson apparatus also includes manually-operated, lip gap adjustment members.
As illustrated by U.S. Pat. Nos. 2,938,231, 3,940,221, 4,281,980, 4,252,519 to Farmer et al, and U.S. Pat. No. 4,726,752, by U.S. Pat. No. 5,020,984 to Cloeren, and by EP Application No. 383,418, an apparatus having a plurality of lip gap-controlling, thermal energy sources spaced apart across the width of the apparatus, and providing for thermally-controlled, lip gap adjustment, is known. The lip gap is narrowed or widened in response to thermal expansion or contraction to provide uniformity of web thickness.
In the type of apparatus exemplified by the '221 patent, an individual cooling means is associated with each lip gap, adjustment assembly According to the Farmer et al patent, a problem with the '221 apparatus is that of "stick-slipping", that is, when a resistance threshold has been overcome, moving suddenly by an amount that may well be excessive.
In the type of apparatus illustrated by the Farmer et al patent, the lip gap is adjusted by temperature adjustment means disposed within a die blade. Contrary to for instance the Richardson patent, which induces localized thermal gradients in the lip, the Farmer et al patent prevents temperature variations from affecting the melt temperature as the melt exits the die, and to this end, is based upon a transversely disposed passage adapted to circulate a heat transfer liquid, as a heat barrier means. The passage is located within the die blade between the temperature adjustment means and the blade lip, so as to maintain the blade lip at a constant temperature (col. 2, lines 33-39). The '980 apparatus includes a heat barrier means for circulating of a cooling medium.
In the type of apparatus exemplified by U.S. Pat. No. 4,726,752 and the Cloeren Patent, common cooling of several lip gap adjustment members is provided to level out various temperature influences including external air currents. In the '752 patent, a cooling medium is forced perpendicular to the adjustment members through channels provided in a cooling block that surrounds the adjustment members. Collectors for supply and discharge of the cooling medium are in fluid communication with the channels. Two sets of channels having opposite flow, may be used.
The EP Patent Application illustrates an extrusion apparatus having an intake/exhaust jacket formed with air passages. Cooling air in the jacket directly impinges upon heaters external to the lip gap adjustment members.
As exemplified by U.S Pat. No. 4,863,361 to Boos, an apparatus having a width-controlling deckle, may include conventional lip heating elements along the lip length. According to the Boos patent, considerable energy is wasted in flowing cooling air around thermally-responsive, lip gap adjustment members.
Accordingly, it may be understood that different approaches have been used to provide an improved apparatus providing uniformity of web thickness. However, these prior efforts continue to be unsatisfactory.
Thus, there continues to be a need for an improved apparatus, and in particular for an improved extrusion apparatus, providing for uniformity of web thickness. Advantageously, thermally-controlled, lip gap adjustment would be improved, and there would be improved consistency in responsiveness to the gap-controlling, thermal energy sources, and faster response time in effecting lip gap adjustment. Moreover, such an improved apparatus would produce a web of given thickness uniformity with reduced lip deformation and reduced lip gap adjustment.